chore(docs): add and refine package-level comments across modules

- Added detailed package-level comments to improve documentation coverage.
- Clarified parameter descriptions, return types, and functionality of exported methods across packages.
- Enhanced overall code readability and API documentation consistency.

internal/usage/logger_plugin.go

@@ -1,3 +1,6 @@
+// Package usage provides usage tracking and logging functionality for the CLI Proxy API server.
+// It includes plugins for monitoring API usage, token consumption, and other metrics
+// to help with observability and billing purposes.
 package usage
 
 import (
@@ -13,12 +16,23 @@ func init() {
 }
 
 // LoggerPlugin outputs every usage record to the application log.
+// It implements the coreusage.Plugin interface to provide usage tracking
+// and logging capabilities for monitoring API consumption.
 type LoggerPlugin struct{}
 
 // NewLoggerPlugin constructs a new logger plugin instance.
+//
+// Returns:
+//   - *LoggerPlugin: A new logger plugin instance
 func NewLoggerPlugin() *LoggerPlugin { return &LoggerPlugin{} }
 
 // HandleUsage implements coreusage.Plugin.
+// It processes usage records by marshaling them to JSON and logging them
+// at debug level for observability purposes.
+//
+// Parameters:
+//   - ctx: The context for the usage record
+//   - record: The usage record to process and log
 func (p *LoggerPlugin) HandleUsage(ctx context.Context, record coreusage.Record) {
 	// Output all relevant fields for observability; keep logging lightweight and non-blocking.
 	data, _ := json.Marshal(record)